The present application discloses an invention that is related, generally and in various embodiments, to optimizing multi-winding transformer manufacturing. More particularly, the present application relates to the optimization of multi-winding transformer manufacturing as applied to transformers for alternating current (AC) motor drives.
Many AC motor drives utilize a rectifier load as a front-end power and current regulator and delivery mechanism. Rectifiers may typically be characterized by a 6-pulse input current waveform having significant harmonic content. In order to limit the harmonic current flowing into the utility (or supply), some transformers use multiple secondary windings. Several rectifiers are used instead of a single fully rated rectifier, and each rectifier is fed from one of the secondary windings of the transformer. The secondary windings may be phase-shifted to provide a multi-pulsed front end that has reduced harmonic content.
An exemplary AC motor drive having multiple rectifiers is described, for example, in U.S. Pat. No. 5,625,545 to Hammond (“the '545 patent”), the contents of which are hereby fully incorporated by reference. As described in the '545 patent, the AC motor drive includes a multi-phase power transformer which has one or more primary winding circuits and a plurality of secondary winding circuits, and each secondary winding circuit is connected to a different power cell. Each power cell contains a rectifier input that is supplied by a dedicated secondary winding. On the output side of the power cells, DC to AC inverters form a series connection to obtain a required voltage that may be needed for each output phase. A 3-phase AC motor drive would include three times the power cells required for each output phase. In general, the desired output voltage of the AC motor drive determines the number of power cells required. Similarly, the number of power cells required determines the number of secondary windings required. Thus, the overall design of the transformer is dependent on the desired output voltage and output current of the AC motor drive.
FIG. 1 illustrates an exemplary AC motor drive 11 which includes a conventional three-phase transformer 13. The transformer includes a primary winding 15 and a plurality of three-phase secondary windings 17, with each winding having specific output voltages and phase angles. On the output side of AC drive 11, each of the three phases of the AC motor is driven by a string of power cells connected in series. In the AC drive of FIG. 1, there are six power cells per phase, labeled A1 through A6, B1 through B6, and C1 through C6, for a total of 18 power cells. It is appreciated that in other implementations, other numbers of cells per phase are possible (e.g., one cell, three cells, nine cells, etc.). In the context of an AC drive or an AC power supply, each power cell is a device which accepts three-phase AC input power, outputs a single-phase AC voltage, and includes an AC-DC rectifier (which may be regenerative), a smoothing filter, and an output DC-to-AC converter.
In the AC drive of FIG. 1, the transformer 13 receives three-phase AC input power from a source, at the points labeled R, S, and T on its primary winding 15. Each power cell receives three-phase AC input power from a dedicated secondary winding 17 of the transformer 13. The eighteen secondary windings 17 have the same nominal voltage, and are arranged in ranks of three, with each rank having one of six specific nominal phase angles. Each secondary winding 17 is directly connected to a power cell, thereby providing each power cell a rectified input voltage and current as discussed above.
When multiple secondary windings are used to feed the power cells, the harmonic contents of the primary side currents are directly related to the number of secondary windings used. The following table illustrates a well known relationship between the number of secondary windings and the effective pulse number on the primary side.
TABLE 1# Secondary WindingsEffective Pulse #Output Voltageper Output Phaseon Primary SideSecondary Winding Phase ShiftsRatings (V)318±20°, 0°4160424±7.5°, ±22.5°48005300°, ±12°, ±24°6000636±5°, ±15°, ±25°80007420°, ±4.3°, ±12.9°, ±21.5°9300848±3.75°, ±11.25°, ±18.75°, ±26.25°10000
Typically in transformer design, each 3-phase secondary winding is delta-connected with taps placed at various positions on the windings to obtain a desired phase shift angle. Various delta windings are illustrated in FIG. 2. A 3-phase secondary winding 10 has taps on individual windings 10A, 10B and 10C positioned such that a phase shift angle of −20° is output by the secondary winding 10. A 3-phase secondary winding 20 has taps on individual windings 20A, 20B and 20C positioned such that a phase shift angle of 0° is output by the secondary winding 20. A 3-phase secondary winding 30 has taps on individual windings 30A, 30B and 30C positioned such that a phase shift angle of +20° is output by the secondary winding 30.
As shown in Table 1, the secondary winding phase shift angles are unique for each number of secondary windings, with the exception of the zero degree phase shift. Thus, to support an AC motor drive that can have anywhere from 3 power cells per output phase (for a total of 9 power cells) to 8 power cells per output phase (for a total of 24 power cells), 16 unique winding phase-shift angles are required.
The transformer design for an AC motor drive supporting variable number of power cells is dependent on the desired voltage and current output of the AC motor drive, as well as the various required phase shift angles. For example, an AC motor drive product family could have rated output currents of 70 A, 100 A, 140 A, 200 A and 260 A (or 5 current levels), and have one rated voltage for each secondary winding, such as that shown in Table 1. Thus, the total number of designs that needs to be supported for this product family would be 480 (16 [phase shift angles]×5 [current levels]×6 [voltage levels]). The large number of unique designs require variations in winding thickness, winding phase-shift designs and iron core cross-sections that need to be generated and maintained during transformer production, resulting in much higher manufacturing costs.